[Skiboot] [PATCH] hw/phb3: Increase AIB TX command credit for DMA read in CAPP DMA mode

[Andrew Donnellan]

When enabling CAPI in DMA mode, set the AIB TX command credits for channel
2 (DMA read) to 28, rather than 1. This significantly improves DMA read
performance in CAPI DMA mode.

Fixes: 5477148a439f ("phb3: Add support for CAPP DMA mode")
Reported-by: John Walthour <jwalthour at us.ibm.com>
Reported-by: Ricardo Mata <ricmata at us.ibm.com>
Reported-by: Michael Perez <perezma at us.ibm.com>
Cc: Ian Munsie <imunsie at au1.ibm.com>
Signed-off-by: Andrew Donnellan <andrew.donnellan at au1.ibm.com>
---
 hw/phb3.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/hw/phb3.c b/hw/phb3.c
index 905fdf4..153eb64 100644
--- a/hw/phb3.c
+++ b/hw/phb3.c
@@ -3352,8 +3352,17 @@ static int64_t enable_capi_mode(struct phb3 *p, uint64_t pe_number, bool dma_mod
 
 	/* aib tx cmd cred */
 	xscom_read(p->chip_id, p->pci_xscom + 0xd, &reg);
-	reg &= ~PPC_BITMASK(42,46);
-	reg |= PPC_BIT(47);
+	if (dma_mode) {
+		/*
+		 * In DMA mode, increase AIB credit value for ch 2 (DMA read)
+		 * for performance reasons
+		 */
+		reg &= ~PPC_BITMASK(42, 47);
+		reg |= PPC_BITMASK(43, 45);
+	} else {
+		reg &= ~PPC_BITMASK(42, 46);
+		reg |= PPC_BIT(47);
+	}
 	xscom_write(p->chip_id, p->pci_xscom + 0xd, reg);
 
 	xscom_write(p->chip_id, p->pci_xscom + 0xc, 0xff00000000000000ull);
-- 
Andrew Donnellan              OzLabs, ADL Canberra
andrew.donnellan at au1.ibm.com  IBM Australia Limited